I find the Cessna dipsticks OK actually. they are always a couple of litres or so pessimistic which is a good thing. I tried the accuracy on a longe range tank 172 with a known amount of fuel and it was remarkably accurate.
My question on the 28 is this; full=182 litres, tabs=133 litres or so but how much fuel have you left when you can just see fuel covering the bottom of the tanks? I've heard two hours to fanstop but obviously you wouldn't want to go on hearsay.
I filled a 161 to the gunnels for a trip to France last year, about two hours each way which I thought would leave me plenty in reserve. When I got back and looked in the tanks I was aghast to see them virtually empty with bare metal showing. I filled up and as I remember put around 130 litres in to brim it, which meant I still had around 50 litres left. Seemed a bit odd to me.